1. Field of the Invention
The present invention generally relates to lithography for semiconductor fabrication, and in particular, to utilizing a liquid medium through which a semiconductor substrate is illuminated during optical lithography. More particularly, the present invention relates to an immersion lithography method and apparatus that employs a contained wafer immersion cell to increase the numerical aperture while minimizing sources of photolithographic light obstruction.
2. Background Information
Lithography, in the context of building integrated circuits (ICs) such as microprocessors and memory chips, is a highly specialized printing process used to put detailed patterns onto silicon wafers. An image containing the desired pattern is projected onto the wafer through a mask defining the pattern. Prior to light projection through the mask, the wafer is coated with a thin layer of photosensitive etch resistant material called “photoresist” or “resist”. For a positive acting resist, for example, the bright parts of the image pattern cause chemical reactions which cause the resist material to become more soluble, and thus dissolve away in a developer liquid, wherein the dark portions of the image remain insoluble. After development, the resist forms a stenciled pattern across the wafer surface which accurately matches the desired mask pattern. Finally, the pattern is permanently transferred onto the wafer surface in an etching process wherein, for example, a chemical etchant is used to etch the portions of the wafer surface not protected by resist; or the pattern may be transferred by ion implantation in which the resist pattern prevents ions from reaching portions of the wafer surface.
With the image resolution of lithography as a limiting factor in the scaling of IC devices, improvements in lithographic components and techniques are critical to the continued development of more advanced and compact ICs. The optical lithography scaling limitation for feature width is expressed by the Rayleigh equation:
  W  =            k      ⁢                          ⁢      λ        NA                  where k is the resolution factor, λ is the wavelength of the exposing radiation, and NA is the numerical aperture. NA may be determined by the acceptance angle of the lens and the index of refraction of the medium surrounding the lens, as follows:NA=n sin α        where n is the index of refraction of the medium between the lens and the image plane and α is the acceptance angle of the lens.        
Faced with problems and limitations relating to using shorter wavelength light sources, optical lithography developers have increasingly looked for ways of increasing the NA of lithography systems. Having low radiation absorption characteristics and for ease of implementation, air has traditionally been used as the transmitting medium. However, having an index of refraction n=1, air as the transmitting medium presents a clear limit to the NA and consequently to the minimum scaling size. Immersion lithography, in which a liquid having a higher index of refraction is used as the medium, is therefore rapidly emerging as an important candidate for upcoming semiconductor lithography applications.
Several immersion lithography techniques are known in the art. One approach, sometimes referred to as the “swimming pool” method, involves wholly or partially submerging the wafer stage, wafer and lens in a pool of immersion fluid, typically water. This technique is referred to as the “bathtub” method when the pool is circulating. Another approach, commonly referred to as the “shower” method, employs nozzles to inject water between the lens and the wafer wherein a suction port for liquid recovery uptakes the injected liquid on the opposite side of the lens after it passes under the lens.
While the foregoing techniques represent progress in the development of this new technology, a number of practical implementation issues remain, including maintaining a pure, non-obstructing transmission medium and compatibility of the tools and wafer with the liquid medium. Purified and degassed water, having a light absorption of 5% at working distances up to 6 mm and an index of refraction n=1.47, may be a suitable medium for immersion lithography. However, problems remain relating to the tendency to form bubbles during the scanning processing. The stage on a lithography exposure tool steps from location to location across a wafer scanning the reticle image for each field. To achieve high throughput, the stage must accelerate rapidly through the immersion fluid, move accurately to the next field location, settle, scan the image and then step to the next location all in a short period of time.
A water medium is susceptible to forming micro bubbles and nano bubbles in the cavitation prone water layer near the moving surfaces, resulting in imaging obstruction and anomalies. Anomalous effects can include absorption, scatter, or an induced birefringent effect with the directional flow of the fluid. Microbubble formation is particularly acute on or adjacent the cavities present in the relatively rough topography at the resist/wafer surface. In addition to problems associated with maintaining purity of the liquid, prior art immersion lithography techniques require substantial redesign of stages for compatibility in a submerged liquid environment requiring significant re-engineering and adding to development costs. Included among the many issues posed by conventional immersion lithography are modifications to lens design and lens casing for compatibility with the resist and immersion liquid, and maintaining immersion liquid properties such as purity, temperature, etc.
From the foregoing, it can be appreciated that a need exists for an improved immersion lithography system and method that substantially increases the NA while minimizing obstruction and distortion of the scanned image. The present invention addresses such a need.